The prior art embraces machines of the type in question, which comprise a dispensing disc rotatable intermittently about a vertical axis and furnished with a ring of cavities, each containing a quantity or portion of tobacco that will correspond to the contents of a single pouch.
The portions are released into the single cavities at a filling station by a hopper containing a supply of powdered tobacco, en masse, treated with flavouring and moisturizing agents.
Downstream of the filling station, the machine comprises skimming means designed to remove any excess tobacco from each of the cavities.
With the disc in rotation, the cavities are carried beyond the skimming means and fed in succession to a station where the single portion of tobacco contained in each cavity is transferred to a further processing stage.
Installed at this same transfer station are pneumatic means comprising a nozzle positioned above the dispensing disc. At each pause in the movement of the disc, a portion of tobacco is ejected by the nozzle from the relative cavity and directed into a duct, of which the mouth lies beneath the disc and in alignment with the nozzle, thence toward a station where the single pouches are formed.
The forming station comprises a tubular element, placed at the outlet of the duct and functioning as a mandrel on which to fashion a tubular envelope of paper wrapping material.
The material in question consists in a continuous web of paper decoiled from a roll and fed in a direction parallel to the axis of the tubular element, which is wrapped progressively around the element and sealed longitudinally.
Beyond the tubular element, the machine is equipped with transverse sealing means of which the operation is synchronized with the transfer of the tobacco portions, in such a way that each successive portion will be sealed in a relative segment of the continuous tubular envelope of wrapping material delimited by two successive transverse seals.
The successive tubular segments of wrapping material, formed as pouches containing respective portions of tobacco, are separated into discrete units through the action of cutting means positioned downstream of the transverse sealing means.
It has been found impossible, utilizing machines of the type outlined above, to guarantee that the quantities of tobacco supplied to the form-fill-and-seal station will be portioned accurately and repeatedly over time.
This is due to the fact, especially when the size of the single portion increases beyond certain limits and the moisture content of the tobacco exceeds given values (typically 30%), that the aforementioned pneumatic ejection means cannot be guaranteed either to remove the contents of the single cavities completely, or to ensure that the portions of tobacco will pass correctly along the connecting duct; consequently, there is a risk that blockages may occur.